1 in 25 year Exceedence

A descriptor of flood magnitude. If there is a 4% chance (that is a 1 in 25 chance) of flow exceeding 100 m3/s occurring in any one year, then 100 m3/s is termed a 1 in 25 year Annual Exceedence Probability flow. One in twenty-five year is not the interval between floods, instead it is the chance of the flood exceeding this flow in any given year. So 1 in 25 year floods can happen two years in a row or twice in any given year. It is based on past data, so the accuracy is unknown for existing and future climates